Gyroscopes are used as sensors for sensing angular velocity or angular acceleration generated by rotation of a movable body. Both mechanical and vibratory gyroscopes are common in the art with vibratory gyroscopes typically formed using MEMS manufacturing processes which generally utilize semiconductor fabrication techniques. Vibratory gyroscopes typically drive a suspended proof mass, such as a tuning fork, etc., to mechanically resonate in a first dimension relative to a support and when the gyroscope experiences a rotation, the Coriolis effect couples energy from the from the excited resonance to an orthogonal (sensed) dimension. A rotation rate (e.g., °/sec) is then determined from the measured amplitude of the coupled energy.
The Coriolis force for a vibratory gyroscope is of the form Fc=2mQ−V, where Q is the angular velocity experienced by the gyroscope, in is the mass weight of the proof mass, and V is the vibratory velocity. Larger mass is therefore advantageous for improved measurement sensitivity. For typical device operation, a change in capacitance between the proof mass and support resulting from the Coriolis force is measured, for example by converting the magnitude of the capacitance to a voltage and/or determining a voltage required to eliminate the movement of the proof mass in the second dimension. In order to improve a sensing capacity of the vibratory gyroscope, capacitive coupling between the suspended proof mass and the support is made large by minimizing the spacing between the proof mass and the support in at least the sensed dimension.
As the artisan will appreciate, it is difficult to fabricate a proof mass having large mass and also a large capacitive coupling factor. To provide adequate capacitance, gap spacing between capacitively coupled faces of the suspended proof mass and the support have been driven down to nanometers using advanced thin film techniques (e.g., conformal depositions, anisotropic plasma etches, etc.). However, pursuit of such thin film micromachining techniques have limited the mass dimensions significantly in the thickness direction (out of the plane of the MEMS device), typically to tens of microns. Even where advanced wafer bonding and transferred substrate techniques are employed, a released proof mass will generally have a thickness no more than a few hundred microns (μm). As such, the proof mass dimensions are limited by the finite surface area available in a given substrate and/or stress in the thin films.
While deep substrate etches have long been utilized in bulk micromachining techniques, as yet, anisotropic etch performance limits lateral dimensions of deep trenches to many tens of microns. As this large lateral dimension significantly limits capacitive coupling, bulk micromachined vibratory gyroscopes have yet to achieve advantageous performance metrics.